1. Field of the Invention
The Phosphoria Formation, which is located in Southeast Idaho and contiguous areas and is of Permian Age, is the largest known deposit of sedimentary phosphate rock in the world. Much of the deposit is deeply buried and can be classified only as a phosphate resource, being uneconomical to mine at the present time. However, due to past tectonic disturbances, numerous surficial exposures of the Phosphoria Formation occur, and these account for the bulk of the phosphate rock mined to date from this deposit. The Phosphoria Formation contains interlayered beds of high grade apatitic phosphate, lower grade phosphatic shales and other non-phosphatic beds of shale and chert. In their original unweathered state, the phosphatic beds contain considerable organic matter and rhombic carbonates, especially dolomite, CaMg(CO.sub.3).sub.2. The phosphate is a fine grained material generally consisting of minute oolites of francolite, a carbonate fluorapatite mineral. Often, and especially in the lower grade shales, the phosphate particles are bonded in a matrix with accessory minerals, such as dolomite and quartz and with phosphatic cements formed by dissolution and subsequent reprecipitation of the source phosphate. In weathered or altered deposits, as found in the surficial deposits currently mined, supra, much of the organic matter and dolomite impurities have been removed in vivo, thereby upgrading the phosphate content, increasing the friability of the rock, and making physical beneficiation more readily accomplished.
As presently practiced, rock from the Phosphoria Formation (henceforward, for the sake of convenience, termed Idaho rock) is selectively mined from the interlayered beds, with only the higher grade phosphate material utilized for wet-process phosphoric acid manufacture. Such rock is usually beneficiated by washing and scrubbing, followed by calcining in air at about 800.degree. C. to remove residual organic matter. Of the lower grade phosphatic rock and shale also mined, some of suitable quality is used in the highly energy intensive and expensive furnace process for producing elemental phosphorus, but the remainder is stockpiled or discarded. As deposits of the surficial altered Idaho rock are becoming exhausted, phosphate producers are increasingly turning to the partially altered or unaltered deposits as sources of their raw material. The increased content of organic matter and dolomite, and the often more indurated nature of the ore makes processing of the unaltered rock more difficult.
Calcination of Idaho rock to remove the organic matter therefrom is presently found to be necessary inasmuch as the presence of such organic material in the rock is deleterious to the subsequent manufacture of phosphoric acid therefrom by the wet process, in which the rock is reacted with a mixture of phosphoric and sulfuric acids followed by filtration of the calcium sulfate formed in the acidulation step to thereby produce a filtrate of phosphoric acid. The presence of such organic material causes undesirable foaming in the acidulation step and can severely hinder filtration of the by-product calcium sulfate from the product acid.
With the increased organic content of the partially altered or unaltered Idaho rock, the calcination step is usually self-sustaining and, as currently practiced, often highly energy inefficient since addition of quenching water is often necessary to moderate the combustion reaction in the calciner. Undesirable formation of sulfide in the calcined product is exacerbated by the high organic content of the more unaltered rock. Particularly objectionable is formation during calcination of what shall be termed "acid-evolved sulfide," the sulfide from which is evolved noxious gases, such as hydrogen sulfide, during acidulation of the rock. In addition to its suspected role in increasing equipment corrosion, acid evolved sulfide, as a precursor of such toxic fumes, is a potential health and environmental hazard.
Appreciable concentrations of soluble magnesium, aluminum, and iron in wet-process phosphoric acid are deleterious to processing and product quality. Presence of these metal ions may cause severe filtration problems, undesirably high acid viscosity, and deposits of sludges during and after processing of the rock to phosphoric acid. The lower grade Idaho phosphate rocks and shales are undesirably rich sources of acid soluble Mg, Al and Fe, and no practical method has heretofore been developed to process this material to phosphoric acid by the wet process. Thus, apart from the small portion used in manufacturing furnace acid, this lower grade Idaho rock, especially in its unaltered form, cannot be used economically with present technology. A significant fraction of the phosphate values in many locations is present as this low grade rock.
Resources of unaltered Idaho rock are vast. For the entire Meade Peak Phosphatic Shale member of the Phosphoria Formation existing within a 350 square mile area of Southeast Idaho, it has been estimated (R. J. Gulbrandsen and D. J. Krier, U.S. Geological Survey Bulletin 1496, 1980) that a total of 125 billion tons of rock with an average P.sub.2 O.sub.5 content of 10.8 percent and an average organic matter content of 8.0 percent is contained therein. Of this rock, these authors estimate that 22.0 billion tons of underground phosphate and a further 5.0 billion tons just underlying the surface contain at least 20 percent P.sub.2 O.sub.5. The average P.sub.2 O.sub.5 content of this higher grade underground rock is estimated at 27.7 percent and the average amount of associated organic matter is estimated to be 6.8 percent. In the face of the rapidly diminishing reserves of the world's high grade phosphate rock, it is apparent that a need exists for improved processes for extracting the phosphate values from this huge resource of unaltered Idaho phosphate rock. More specifically, a need exists for a thermal process which will improve the quality of the phosphate material by removing the organic matter, by minimizing undesirable sulfide formation, and by removing or deactivating most of the soluble Mg, Al, and Fe species, such as to convert substantially all the phosphate values to a product which may be satisfactorily acidulated to wet-process phosphoric acid. Furthermore, the need to fully utilize the appreciable organic content of this rock, possibly as a fuel, is a consideration of the instant invention.